1. Field of the Invention
The invention relates to a method of treating plutonium and/or uranyl nitrate in a nitric acid starting solution to produce a nitrate in compact solid form which can with greater safety be stored or transported as a solid body until further processes.
2. Description of the Prior Art
U.S. Pat. No. 4,235,740 discloses heating and concentrating a nitric acid starting solution. For further processing, such heated and concentrated starting solution can be drained into a precipitation device, in which for instance ammonium carbonate solution is admixed and ammonia and excess CO.sub.2 are dosed in, so that finally (NH.sub.4).sub.4 PuO.sub.2 (CO.sub.3).sub.3 or (NH.sub.4).sub.4 (Pu/U)O.sub.2 (CO.sub.3).sub.3 is precipitated. This precipitated product can be calcined in accordance with U.S. Pat. No. 4,235,740 in a reducing atmosphere and subsequently oxidized, while air is being supplied, to form powdered PuO.sub.2 or (Pu/U)O.sub.2 for instance with stoichiometric oxygen content. The powdered PuO.sub.2 or (Pu/U)O.sub.2 can be sintered to form nuclear fuel bodies for nuclear reactor elements.
As is well known, plutonium or uranium is produced as plutonium nitrate or uranyl nitrate in a nitric acid liquid solution if irradiated, burned-up uranium nuclear fuel is reprocessed.
The plutonium and the uranium can then be stored and/or transported in the form of this nitric-acid liquid solution up to reprocessing the metal into oxide. This procedure however, is extremely expensive, particularly since the nitric acid solution can contain only about 200 to 400 grams plutonium or uranium per liter, so that storage or transport tanks as well as auxiliary piping and pumps of considerable volume are required which must be protected against leakage of the solution.
It is true that the plutonium or the uranium in the liquid nitric acid solution can also be converted immediately after this solution is produced at the reprocessing site of the burned-up nuclear fuel, for instance to powdered plutonium oxide or uranium oxide, so that only this plutonium or uranium oxide in powder form need be stored and/or transported. When plutonium is stored, however, its isotope with the atom number 241 decays radioactively to americium-241 which emits hard .gamma. rays and is also a neutron poison. If the plutonium is stored too long, the americium-241 content becomes excessively high and must therefore be removed in many cases before the stored plutonium can be processed further. For this purpose, the plutonium must be present dissolved in nitric acid.
Plutonium oxide (PuO.sub.2), however, can be dissolved completely in nitric acid only if special measures are taken, such as the addition of hydrofluoric acid. If, therefore, stored plutonium oxide in powder form is processed further into nuclear fuel bodies, it must first be dissolved in an HNO.sub.3 /HF-mixture, the americium-241 must be separated and the residual solution again converted into plutonium oxide. The facilities required for the dissolution must be made of highly corrosion-resistant material and are therefore elaborate and expensive.
It is furthermore impractical to convert plutonium and/or uranyl nitrate solution produced in reprocessing of burned-up uranium nuclear fuel immediately at the reprocessing site into powdered plutonium or uranium oxide, because, for example, the grain size and the flowability of the oxide powders are determined by the conversion process used. It is, therefore, more advantageous as a rule for the manufacturers of fuel asesmblies to carry out the conversion step to oxide powder themselves, since, the grain size and the flowability can then be better adapted to the individual requirements of the fuel assembly manufacturers.